The present invention relates generally to reactive ion etching endpoint detection, and more particularly, to endpoint detection via laser induced fluorescence.
Reactive ion etching or plasma etching is now typically used to delineate fine line patterns and trenches in thin films of either insulators, semiconductors, or metals by means of the removal of portions of these films in a plasma discharge. The wafer to be etched is placed in a plasma chamber into which a gas mixture is directed at a reduced pressure. In the presence of a plasma discharge generated by the application of RF energy, reactive species are generated from the feed gas by processes such as dissociative or impact ionization. Portions of the thin film are removed by chemical reaction between the reactive species and the film, as well as by bombardment of the film by ions present in the plasma. The gaseous reaction products formed by the chemical reactions and the ion bombardment of the film are continuously removed from the chamber using a vacuum pump.
One of the problems encountered in the use of reactive ion etching is the insufficient reproducibility of the etching rate. In part, this etch reproducibility problem is caused by variations in the plasma composition due to the time dependent presence of etch products, difficulties in completely controlling the surface temperature of the wafer or wafers to be etched, and batch-to-batch variation in the quantity of material to be etched, or the load. Because of this variation in the etching rate, reactive ion etching in many cases requires monitoring to detect the completion of the etching process. In this regard, it is important to detect end of the etching process in order to terminate the etch before over-etching occurs in the sublayer below the layer being etched. Such over etching is detrimental not only because it attacks the substrate or sublayer below the layer being etched, but also because it causes undercutting of the etch pattern, thereby altering the dimensions of the desired features in the etched layer.
In a typical endpoint detection scheme, a majority chemical species from the layer being etched enters the etching plasma and is observed by monitoring a relevant spectral line for that majority species as the etching process consumes the layer being etched. The time to terminate the process is inferred from changes in the intensity of this monitored majority species spectral line. The monitoring of the majority species coming from the etched layer works well in many situations where the etched layer and the sublayer therebelow are composed of different materials. However, when the composition of the etched layer and its sublayer therebelow are similar or the same, then monitoring of the majority species from the etched layer will not provide a determination of the etch endpoint. A similar problem is encountered when techniques are utilized to compensate for etch loading non-uniformities. For example, an aluminum film on a wafer is many times etched by disposing the wafer on a high purity aluminum target to thereby prevent a sudden large excess of etching species near the end of the etch process which would cause an attendant undercutting of the aluminum film. However, the use of this aluminum target prevents the determination of the etch endpoint by monitoring the majority aluminum species.
The invention as claimed is intended to remedy the above-described drawbacks. Specifically, it solves the problem of detecting the etching endpoint through a film when the majority species in the etched film and in the sublayer therebelow are the same. Additionally, the present invention solves the problem of detecting the etching endpoint through a film when the majority species in that etched film is the same as a specially designed target disposed adjacent to the wafer being etched.